The present invention relates to the field of optical shaft encoders, and more particularly to an optical shaft encoder which provides higher angular resolution for a given number of detectors than has been previously possible.
Optical shaft encoders are well known for their use in determining the angular position of a rotating shaft. Such optical shaft encoders are used in various types of machinery and machine tools where information concerning the precise angular relationship between a shaft and another component is needed.
Optical shaft encoders generally consist of an optical shutter, such as a disk or drum, which is rigidly attached to the shaft whose position is to be determined. The optical shutter is used to modulate the transmission of light between a light emitter (e.g. a light emitting diode) and a light detector (e.g. a photodiode or phototransistor). This modulation may be transmissive, in which case the optical shutter has a pattern of slots formed on its surface and the light emitter and light detector are arranged on opposite sides of the optical shutter. Alternatively, the modulation may be reflective, in which case the optical shutter has a pattern of reflective (e.g. white) and absorptive (e.g. black) areas on its surface and the light emitter and light detector are arranged adjacent one another on the same side of the optical shutter.
In general, the resolution of a optical shaft encoder depends on the number of light emitting and light detecting devices used. For example, using one emitter and one detector, whose output will be either a "1"(on) or "0"(off), will give an angular resolution of 1/2 a revolution of the shaft. If two emitters and two detectors are used, the detector outputs can have up to four different states (e.g. 00, 01, 10, or 11) and thus will be able to resolve 1/4 of a revolution. Three emitters and three detectors can have eight different states and resolve 1/8 of a revolution; four emitters and four detectors can have sixteen different states and resolve 1/16 of a revolution, etc.
Optical shaft encoders have also been proposed for use in detecting the position of the shafts which drive a clock-dial type register display such as used in a gas, water, or electricity meter. Such a display usually consists of a dial face similar to that used on a clock and having the numerals 0-9 printed thereon. A pointer is removably attached to each shaft, and each shaft is in turn linked to the metering mechanism of the meter. The shaft and pointer are normally rotated by an amount that is proportional to the amount of billable commodity (e.g. gas, water, electricity) being monitored by the meter.
Since a clock-dial type register display for use in such a meter requires an angular resolution of at least 1/10 of a revolution, conventional optical shaft encoders, constructed as described above, would require four light emitters and four light detectors (i.e. producing a total of 2.sup.4 =16 possible states) in order to resolve the 1/10 of a revolution necessary for proper reading of such dials.
From the viewpoint of a utility company, it would be beneficial to have a way of automatically reading the position of each of the dial pointers, since the manual reading of such meter displays is slow and prone to error. However, any attachment which might be added to a clock-dial type register would need to be simple, compact and inexpensive in order to meet the needs of a utility.